The present invention relates to a dichroic mirror device to separate or synthesize a red light (R), a green light (G) and a blue light (B), which are used for a projector such as a liquid crystal projector.
Liquid crystal projectors, CRT projectors and CCD cameras are accommodated in rear projection televisions, front projection televisions or television cameras. Liquid crystal panels used for the above colored lights R, G or B or projectors employing a CRT require a mechanism to separate or synthesize lights. A conventional device for light separation/synthesis uses a glass prism. However, the glass prism is actually used only for 1- to 2-inch liquid crystal panels. If employing a glass prism for a larger liquid crystal panel or a CRT, the glass prism has to be very large and thus it will cost a lot of money. Furthermore, there may arise the problem of retardation (irregular phase difference), when a large glass prism is used.
Another conventional device has then been proposed, which uses three dichroic mirrors instead of a glass prism. FIG. 13 shows a so-called conventional cross-type three color separation/synthesis mirror device 4. The device comprises a dichroic mirror 1 and two other dichroic mirrors 2 and 3 holding the mirror 1 therebetween, which forms an X-shape. A blue light B is emitted from a CRT 5, a green light G from a CRT 6 and a red light R from a CRT 7, respectively. Those three colored lights are synthesized by the separation/synthesis mirror device 4, pass through a lens 8 and are projected to the screen (not shown in the figure) to form an image.
However, a light passing through the joint portion 9 goes along a different path from that of a light passing through other portions of the device, which affects the image quality. For instance, a double image appears around the center of the screen, the image gets blurred or it lacks clearness, resulting in decrease in number of scanning lines.
The other problem is an out-of-focus image and a decline in image contrast caused by an increase in the astigmatism. The astigmatism increases because the dichroic mirrors 1 to 3 are placed obliquely to the CRT's 5 to 7.